Picture #1 from Copilot Ai:

Q: "How should pieces of duct for a range hood be attached together and sealed?"

A: "How to seal the joints (interior ductwork)

Sources agree on two acceptable materials:

1. UL‑listed aluminum foil tape (UL 181)

• Apply foil tape to every seam and joint.
• Press firmly to eliminate bubbles and ensure an airtight bond.
• Standard “duct tape” is not acceptable—its adhesive fails under heat and grease.

2. Duct mastic (optional but excellent)

• A thick, paint‑on sealant that cures into a flexible, permanent barrier.
• Recommended for long‑term airtightness and high‑pressure joints.
• Apply with a brush; embed fiberglass mesh if gaps are large.

Most homeowners use foil tape alone; pros often use foil tape + mastic for maximum durability.

• Master Flow 10.5 oz. Water Based Duct Sealant Tube

Picture #2:

• Master Flow 6 in. 90 Deg. Round Adjustable Elbow

1. If you assemble from the wall interface downward to the hood, you can reach up into the first duct section and tape from the inside using aluminum foil tape. Silicone caulk can also be used. Your Authority Having Jurisdiction (Code Enforcement Officer) has the last word. Your sealing goal includes keeping the sliding interfaces from separating.

2. You may need a custom piece, but it might be possible to cut one from another elbow.

3. If the hole is misaligned with the duct, a little more cabinet back will have to be removed.

Note: duct has to be galvanized steel and not aluminum.

So you are venting out the wall, but going up into the cabinet first?

Undercabinet range hoods can typically be ducted either out the back up out the top. Your hood will be more efficient (and the cabinet above will be more roomy) if you go straight out through the wall.

"If you assemble from the wall interface downward to the hood, you can reach up into the first duct section and tape from the inside using aluminum foil tape. Silicone caulk can also be used"

@kaseki. The aluminum foil tape or silicone caulk can be applied on the INSIDE of the joint. Is this correct?

I thought the aluminum foil should be applied along the exterior joint and silicone inside the duct.

Thanks

I found the inside-the-duct tape idea in a YouTube video that I believe was created by an agency of Vent-a-Hood. I didn't keep a reference link. The idea is to keep the grease and greasy air inside the duct. Flame spread to the outside of the duct should be adequately suppressed by the overlap of the inner crinkled and outer smooth duct portions. Band clamps (à la Fantech) can aid this, but aluminum duct tape isn't intended to be a fire stop.

(Note, also, that there may be a required gap between duct and combustible surfaces, and sheetrock (gypsum board) is included in the list of combustibles. Cabinet surfaces also. Your hood installation instructions should provide any needed requirements. Normally the hood baffles or mesh or VaH squirrel cage would be the fire stop for a cooking fire. )

If a fire occurs with presumed damage to internal aluminum duct tape, one would want to inspect, reseal, and potentially replace the duct on general principles.